This invention relates to an apparatus for measuring the amount of positional deviation of a print which is caused when a recording sheet is conveyed in a copying machine or printer, and more particularly to an apparatus for measuring the amount of positional deviation of a color print.
FIG. 14 shows the arrangement of an apparatus for measuring the amount of positional deviation of a recording sheet which has been disclosed, for instance, by Unexamined Japanese Patent Publication No. 242343/1985. As shown in FIG. 14, a recording sheet 1 on which cross marks 1A are recorded is placed on a table 2A which is moved on a base 2 in the directions of the arrow Y. A gate-shaped structural member 5 is provided above the table 2A, and is provided with a moving unit 5A which is movable in the directions of the arrow X, and the moving unit 5A has a laser unit 3, the lower end of which is coupled to a measuring unit 4. The measuring unit 4 is shown in FIG. 15 in detail. In FIG. 15, a light source 3A is a He-Ne laser, and its output laser beam is converted into a parallel laser beam by an optical system 6, which is applied to the recording sheet 1. The laser beam reflected from the surface of the recording sheet 1 is applied through an optical system 6A to a photo-electric conversion element 7; that is, it is detected by the photo-electric conversion element 7. Referring back to FIG. 14, a control unit 8 operates to move the table 2A and the moving unit 5A in accordance with the data, which have been set by a computer 9, in the directions of the arrows X and Y. The control unit 8 has means (not shown) for inputting data into the computer 9 which represent the positions of the table 2A and the laser unit 3. The control unit 8 further operates to control the operation of the laser source 3A and to subject the output of the photo-electric conversion element 7 to A/D (analog-to-digital) conversion and apply the output thus processed to the computer 9.
In the conventional apparatus thus constructed, the recording sheet 1, on which the cross marks 1A as shown in FIG. 16 have been recorded, is fixedly placed on the table 2A, and the positions of the cross marks 1A are detected with the detecting unit 4. The data on the cross marks thus detected are applied to the computer 9. In the computer 9, those data are compared with the data on correct position which have been inputted in advance, whereby the amount of deviation from the correct position is detected.
With the conventional apparatus for measuring the amount of positional deviation of a recording sheet, the recording sheet is not even depending on its surface condition; that is, of the recording sheet, the border line between the printed region or inked region and the background color region is not a straight line but a curved line irregular to about 50 .mu.m. Accordingly, the position detected with the detecting unit 4 is large in error; that is, it is impossible to measure the amount of positional deviation with high accuracy.
The conventional apparatus is further disadvantageous in that, since it is necessary to move the measuring unit to each of the measuring positions with a moving system, the mechanical error of the moving system is added to the results of measurement.